The emergence of magnetic random access memory (MRAM) is a promising step for the development of both long-term and short-term data storage. MRAM has the benefit of being non-volatile, while having a lower energy consumption and faster read and write time than Flash memory. MRAM also has a lower energy consumption than the commonly used volatile memories dynamic RAM (DRAM) and static RAM (SRAM), with a read and write time faster than that of DRAM.
A conventional MRAM cell comprises a magnetic element having a ferromagnetic free layer and a ferromagnetic pinned layer, separated by a non-magnetic layer. The pinned layer has a relatively high coercivity, so that its magnetisation remains fixed upon the application of a writing magnetic field. The free layer has a relatively low coercivity, so that its magnetisation can be changed upon application of the writing magnetic field.
To write to the MRAM cell, the writing magnetic field is applied to switch the magnetisation of the free layer to be either parallel or anti-parallel to the pinned layer. The free layer exhibits magnetic hysteresis, thus its magnetisation remains unchanged when the magnetic field is removed. This results in a non-volatile memory.
To read the state of the MRAM cell, a small current is driven through the magnetic element. The magnetoresistance of the magnetic element will be higher when the magnetisations of the free layer and the pinned layer are antiparallel, than when the magnetisations of the free layer and the pinned layer are parallel. In this way, the state of the magnetic element can be determined by measuring its resistance.
A conventional MRAM is described in “Recent Developments in Magnetic Tunnel Junction MRAM” by S. Tehrani et al., p.2752-2757, IEEE Transactions on Magnetics, Vol. 36, No. 5 (September 2000).
Such a conventional MRAM suffers from the disadvantage that as the size of the MRAM cell decreases, the magnetic field required to switch the magnetisation of the free layer increases. Therefore, the power consumption of the device increases as the cell size decreases.
Another technique used to write to a magnetic element is spin-transfer-torque (STT) switching. STT switching is described in “Current-driven Excitation of Magnetic Multilayers” by J. C. Slonczewski, p.9353, Phys. Rev. B, Vol. 54 (1996). To switch the magnetisation of the free layer, instead of applying a magnetic field, a current is driven through the magnetic element perpendicular to the plane of the free and pinned layers. This can result in the injection of spin-polarised electrons into the free layer, either by electrons flowing through the pinned layer, when current is driven from the free layer to the pinned layer, or by electrons scattering from the pinned layer 85, when current is driven from the pinned layer to the free layer.
When spin polarised electrons are injected into the free layer, their spin angular momentum interacts with the magnetic moments in the free layer. The electrons transfer a portion of their angular momentum to the free layer. This results in switching the magnetisation of the free layer when the spin-polarised current is large enough.
An MRAM utilising STT switching is described in “Highly scalable MRAM using field assisted current induced switching” by W. C. Jeong et al., p.184, 2005 Symposium on VLSI Technology Digest of Technical Papers.
The current required for STT switching decreases as the cell size decreases. Therefore, high density MRAM can be realised with STT switching. For DC current, the threshold current density for STT switching depends on material constants such as the saturation magnetisation, Gilbert's damping constant, and the spin polarisations of both the pinned and free layers. However, the required current for a nano-second pulse is much larger than the DC threshold current. It has been shown that the required current in the nano-second regime is given byI=Ic0(1+C·tp−1)  (1)where C is a constant, and Ic0 is the DC threshold current. According to equation (1) above, the current required to switch the magnetisation for a 1 ns pulse is four times the DC threshold current. Therefore, STT switching MRAMs having a fast write time will have large power consumption.
Another MRAM is described in “A Novel Non-volatile Memory with Spin Torque Transfer Magnetization Switching: Spin-RAM” by M. Hosomi et al., p. 19.1, IEEE International Electronic Device Meeting 2005, which shows that the current required for STT switching increases significantly in the nano-second regime.
In “Precharging strategy to accelerate spin-transfer switching below the nanosecond” by T. Devolder et al., Appl. Phys. Lett., 86, pp. 062505 (2005), an MRAM is described in which a DC bias current is applied in addition to a short RF current pulse. This can reduce the current required for STT switching in the nano-second regime. However, using a DC bias current dramatically increases the total power consumption of the MRAM.
In addition, MRAMs utilising STT switching have an intrinsic probability distribution in the switching current. This is caused by the distribution of the initial magnetisation direction of the free layer due to thermal fluctuations. To ensure error free switching, in all the MRAM cells, the switching current is increased. In addition, the difference between the writing current and the reading current is decreased.
The present invention seeks to ameliorate at least some of the above-identified problems.